The present invention relates generally to turbine buckets and, more particularly, to a turbine bucket incorporating an optimized cooling circuit with modified cooling hole sizes and positions in an effort to maximize cooling ability and ensure a longer useful life.
In gas turbine engines and the like, a turbine operated by burning gases drives a compressor which furnishes air to a combustor. Such turbine engines operate at relatively high temperatures. The capacity of such an engine is limited to a large extent by the ability of the material from which the turbine blades (sometimes referred to herein as “buckets”) are made to withstand thermal stresses which develop at such relatively high operating temperatures. The problem may be particularly severe in an industrial gas turbine engine because of the relatively large size of the turbine blades.
To enable higher operating temperatures and increased engine efficiency without risking blade failure, hollow, convectively-cooled turbine blades are frequently utilized. Such blades generally have interior passageways which provide flow passages to ensure efficient cooling, whereby all the portions of the blades may be maintained at relatively uniform temperatures.
While smooth-bore passages have been utilized, turbulence promoters, e.g., turbulators, are also used in many gas turbine buckets to enhance the internal heat transfer coefficient. The heat transfer enhancement can be as high as 2.5 times that of smooth-bore passages for the same cooling flow rate. Turbulators conventionally comprise internal ridges or roughened surfaces along the interior surfaces of the cooling passages and are typically cast inside the cooling passages using ceramic cores and/or STEM (shaped tube electrochemical machining) drilling.
In earlier attempts to improve the original four-hole stage 2 bucket, additional cooling was introduced by adding cooling holes and incorporating turbulators to increase the heat transfer coefficients at certain locations. The resulting seven-hole bucket was to be in uprated machines firing at 2075° F. Due to unbalanced stack issues, the seven-hole bucket design was severely local creep limited in its trailing edge.
A redesigned baseline six-hole bucket was better balanced and also incorporated turbulation; however, in an attempt to recover some performance, the cooling flow through the component was drastically reduced, leading to bulk creep life limitations.